The nation’s tax collectors were instructed for two years on how to conceal evidence obtained by a secret unit inside the Drug Enforcement Administration cooperating with the National Security Agency.

An Internal Revenue Service manual posted online in 2005 and removed in 2007 instructed agents on how to conceal classified evidence forwarded by the DEA’s Special Operations Division (SOD) in investigations of Americans, Reuters reports.

Reuters reported Monday that the SOD forwards tips acquired from the National Security Agency, “wiretaps by foreign governments, court-approved domestic wiretaps,” and a DEA phone and Internet database called DICE. These tips go to federal agents and local law enforcement officers.

Officials at both the DEA and the NSA stressed that the DICE database and the NSA database currently at the center of controversy were different databases.

Recipients of the information are then instructed to engage in a decades-old law enforcement technique called “parallel construction;” the tipped-off agent might use other non-secret means to justify beginning an investigation into a suspect and conceal the origin of the evidence.

On Wednesday, Reuters reported that the IRS manual — no longer available online — instructed agents to use the evidence as leads, but find new “independent” evidence to justify the investigation.

Evidence obtained from SOD, which is closely guarded by the DOJ, cannot be directly used in an investigation.

The IRS already came under fire earlier this year for having a policy that allowed its criminal division to engage in the warrantless search of a suspect’s electronic communications. That policy was amended following pressure from members of Congress.

The parallel construction technique advised by the SOD is legal, but not without its critics.

Both Republican Congressman Mike Rogers, Chairman of the House Permanent Select Committee on Intelligence, and Kentucky Republican Senator Rand Paul have both spoken critically of the program.

Rogers, a former FBI agent, told talk show host Mike Huckabee, “If they’re recreating a trail, that’s wrong and we’re going to have to do something about it.”

Paul, who has staked his claim on the defense of the Constitution, also expressed concern over the technique, noting that the protection of individual liberty was just as important a function of the government as national security.

Two dozen federal government agencies comprise the unit, which was formed in 1994, including the Federal Bureau of Investigation, Central Intelligence Agency, Department of Homeland Security, National Security Agency and Internal Revenue Service.